1. Field of the Invention
This invention relates to a forming roll which is a rotatable cylindrical roll positioned in the forming section of a Fourdrinier papermaking machine and supported above the forming medium in contact with one surface of a web being formed to sandwich between the roll surface and the forming medium the constituent material which forms the web. The forming roll is particularly useful in the actual formation of a paper web as well as in the improvement in the formation quality of the paper web.
2. Description of the Prior Art
In the typical Fourdrinier papermaking machine, an aqueous suspension of fibers, called the "stock" is flowed from a headbox onto a traveling Fourdrinier wire or medium, generally a woven belt of wire and/or synthetic material, to form a continuous sheet of paper or paper-like material. In this connection, the expression "paper or paper-like material" is used in a broad or generic sense and is intended to include such items as paper, kraft, board, pulp sheets and non-woven sheet-like structures. As the stock travels along on the Fourdrinier wire, formation of a paper web occurs, as much of the water content of the stock is removed by draining. Water removal is enhanced by the use of such well-known devices as hydrofoils, table rolls and/or suction devices. Subsequent to the formation of the paper web, the paper web may be caused to engage a dandy roll to improve both the formation of the sheet and the quality of its surface. While water is being removed from the stock and the paper web is being formed and thickened, the consistency of the constituent material varies from the consistency of the stock at the headbox, which may be about 0.5 to 1%, to the consistency of the fully-formed paper web, about 2 to 2.5%, and finally as it engages the dandy roll, usually about 3% to 4%. It is generally understood that until such time as the paper web is fully-formed at a consistency of about 2 to 2.5%, the forming medium carries a partially-formed web with some still unformed stock above it. The terms "web/stock sandwich" and "constituent material" are used interchangeably herein to identify the combined partially-formed web having the unformed stock above it.
In recent years, there has been a desire to increase the efficiency of the Fourdrinier machine while at the same time increase the quality of the paper being formed. In attempting to meet these objectives, efforts have been made to modify existing machines. For example, as noted herein, it has long been known to improve the quality of the paper being formed in the forming section of a Fourdrinier machine by positioning a dandy roll above the forming medium and in contact with the paper web at a point where the paper web has already been formed and the average consistency of such web is about 3% to 4%. Usually, suction devices such as suction boxes have been positioned upstream and downstream of the dandy roll to facilitate the use thereof. Efforts to improve the results obtained by use of such dandy rolls have included the use of rolls having greater diameters than the diameter of a standard dandy roll in order to increase the surface area of the roll against which the paper web is urged by the forming medium. In fact, dandy rolls having a diameter of up to six feet are known to be in use. However, there has been a tendency in modern Fourdrinier machines to increase machine speed, and the combination of greater machine speed and dandy roll diameter has not been entirely satisfactory. For example, in such applications there has been a tendency for an excessive amount of water to be flung back upon the web when it is separated from the dandy roll during the papermaking operation.
In a further effort to increase the usefulness of the dandy roll in improving the formation quality of paper, a partially-formed web with some still unformed stock above it at an average material consistency in the 1.5 to 2% range has been known to engage a dandy roll dipping significantly, 3 to 4 inches, down into the forming medium to increase the surface area of roll against which the web/stock sandwich is urged by the forming medium. To facilitate operation of such an embodiment, the suction boxes positioned upstream and downstream of the dandy roll have been tilted so that the surfaces of the suction box covers substantially conform to the path traversed by the slightly curving forming medium in the vicinity of the dandy roll. Such structure has been useful in improving the formation quality of the paper web at papermaking machine speeds not in excess of 1000 f.p.m. However, at greater speeds there is a tendency for the disruption, due to its momentum, of the stock over the partially-formed web in the vicinity where it first engages the dandy roll.
In another known apparatus described in West German patent application DE No. 3142O54 A1 in the name of Bubik et al, a fiber suspension is delivered upon a traveling Fourdrinier wire. The suspension is subsequently sandwiched between the wire and the porous traveling surface of a dewatering element such as a dewatering cylinder positioned above the wire. To effect such sandwiching, the wire and suspension are guided downward over a first element such as a first guiding cylinder or roll positioned beneath the wire and upstream of the dewatering cylinder, and then guided upward over a second element such as a second guiding cylinder or roll positioned beneath the wire and downstream of the dewatering cylinder. In this manner the wire travels in essentially the same plane before the first element and after the second element and is caused to deviate from such plane at the dewatering element. The area beneath the wire extending from the position where the wire disengages the first element to the position of deepest penetration of the wire by the dewatering element, is provided with a vacuum zone. The Bubik et al application states that by using such structure, back flow originating in the upper portion of the fiber suspension is prevented at the entrance of the suspension onto the dewatering element. In addition, it is stated that such structure prevents a strong back flow in the zone between the first element and the dewatering element, and that the prevention of such back flow prevents damage to the fiber formation. In the foregoing application, recognition is made of the fact that in the absence of the use of vacuum in such vacuum zone, the upper layer of the fiber suspension will impact against the dewatering element at a relatively steep angle and at a relatively high impact point as a result of the centrifugal force in operation upon the suspension, and that such impact causes harmful back flow in the upper layer of the fiber suspension which can damage the formation of the stock fibers in this upper region. To further assist in the operation of the apparatus described in the foregoing application, it is stated that a pressure area is positioned in a zone generally existing between the deepest penetration of the wire by the dewatering element and the second element.
The use of such a vacuum zone in the apparatus of Bubik et al is not believed to be adequately satisfactory in preventing surface disruption of the fiber suspension at machine speeds in excess of 800 f.p.m. In addition, any advantage resulting from the dewatering associated with the downstream pressure zone is believed to be offset by the problem of maintaining the fiber suspension upon the Fourdrinier wire as the wire passes over the second element and continues on to the next phase of the papermaking operation in the presence of such pressure.
Other efforts directed to the modernization of existing machinery have included the conversion of standard Fourdrinier machines into what has been referred to in the industry as "hybrid formers." Generally, a hybrid former includes a typical Fourdrinier section of a papermaking machine having a twin-wire former installed on top of the existing forming medium. More particularly, in such a structure an endless belt or wire is placed above the forming medium and is caused to travel around a plurality of rolls to squeeze the web against the forming medium. The upper wire and the paper web travel around a portion of a large diameter roll and then arcs around additional rolls, suction being applied to the forming medium after it leaves the last roll, to assure that the paper web adheres to the forming medium and continues through the papermaking process.
The installation of a hybrid former involves the use of relatively complex and expensive apparatus. Such apparatus may include multiple rolls such as those required for guiding and stretching the upper belt or wire. In addition, various dewatering devices need be provided, as well as shower means and the like for cleaning the upper belt or wire. Installation of a hybrid former also requires proper alignment on its support structure to assure proper operation, a task which is complicated due to the excessive weight of the twin wire apparatus which may be as high as 110 tons.
Accordingly, it is an object of this invention to provide a means whereby the efficiency of a Fourdrinier machine and the quality of the product produced thereon may be increased without the use of complex and costly apparatus, and with minimum modification of existing machinery.
Another object of this invention is to increase the efficiency of a Fourdrinier machine and the quality of the product produced thereon without the need for a top wire or fabric loop traveling about a plurality of rolls and other associated apparatus positioned above the forming medium, and without the alignment problems associated with use of such a top wire.
Yet another object of this invention is to increase Fourdrinier machine efficiency and paper quality by the use of relatively simple apparatus capable of being located at a position above the forming medium where the average consistency of the constituent material is about double that of the stock consistency at the headbox.
Still another object of this invention is to increase the surface area of a forming roll against which the constituent material is sandwiched by a forming medium without requiring an increase in the diameter of the forming roll.
A further object of this invention is to obtain the advantages of a hybrid former without a top wire or fabric loop and the plurality of rolls associated therewith by positioning a forming roll in the forming section of a Fourdrinier machine above the forming medium where the average consistency of the constituent material is less than about 2%.
Yet a further object of this invention is to achieve all the foregoing objects on a Fourdrinier machine operating at any machine speed including a speed in excess of 800 feet per minute (f.p.m.).
Another object of this invention is to achieve all of the foregoing objects on a Fourdrinier machine in which the degree of surface area of a forming roll against which the constituent material is sandwiched by a forming medium is substantially increased without the use of vacuum augmenting apparatus beneath the forming medium in the general area preceding where the constituent material first engages the forming roll.
Yet another object of this invention is to achieve all of the foregoing objects on a Fourdrinier machine in which the degree of surface area of a forming roll against which the constituent material is sandwiched by a forming medium is substantially increased without the use of a pressure zone beneath the forming medium in the general area where the paper web disengages the forming roll.
A further object of this invention is to achieve all of the foregoing objects on a Fourdrinier machine while substantially eliminating disruption of the product being processed in the general area where the constituent material engages and the paper web disengages the forming roll.